Field Of The Invention
The present invention relates to lithium batteries and, more particularly, to a lithium battery having a depletion gauge or indicator.
Lithium batteries have become well-known for their long life and suitability as power sources for cardiac pacers and nerve stimulators. Normally these batteries utilize a lithium anode, a cathode formed of cupric sulfide or silver chromate, and an electrolytic solution.
With the long life associated with such batteries and due to the fact that such batteries are generally implanted within the human body, it is desirable to have a method for providing an indication of the level of discharge of the battery. This is particularly important in life sustaining devices, such as cardiac pacers, in order to provide adequate time for the replacement of the device, or at least the battery, before failure occurs.
An important feature of the present invention is that of providing a means within a battery, or energy cell, for providing a positive indication of the level of discharge of the battery, or conversely, to provide a positive indication of the remaining life of the battery.
It is important that a positive indication of remaining battery life be provided regardless of the type of service the battery has been providing. In addition, it is important that the depletion gauge or indicator be compatible with the present construction and configuration of lithium batteries utilized in implanted devices, such as cardiac pacers and nerve stimulators.
In certain types of lithium batteries, the internal resistance of the battery increases linearly with the discharge of the battery. With this type of battery, the output voltage provides as satisfactory indication of the remaining life of the battery.
With certain other types of lithium batteries, the voltage output remains approximately constant until the battery is substantially discharged. With still other types of lithium batteries, such as the lithim-cupric sulfide battery, an end-of-life indication may be obtained by taking advantage of the two stages of discharge i.e.: EQU Cu.sup.+2 .fwdarw.Cu.sup.+1 E=2.1 volts EQU Cu.sup.+1 .fwdarw.Cu.degree.E=1.7 volts
By adjusting the ratio of lithium to cupric sulfide, it is possible to control the ratio of these two reactions. Accordingly, when the transition from the higher voltage to the lower voltage occurs, this change in voltage provides an indication that a predetermined level of discharge of the cell has occurred. In batteries used with cardiac pacers, the length of the second voltage level is adjusted to be approximately 10 percent of the first voltage level. One problem with these dual voltage level systems is that once the second level is obtained, the battery may then last from three months to one year depending upon the load applied to the battery at the second voltage level.
With cardiac pacers of more modern design which include complex programmable capabilities, the drain on the battery may vary over a broad range. It therefore becomes desirable to have an additional means for evaluating the residual capacity of the battery at a time early enough to establish orderly plans for the replacement of the device. Also, as greater loads are placed upon batteries, the effectiveness of the dual level indicator system is substantially reduced.